Slab waveguide elements introduced into conventional optical systems can provide unique methods of processing optical beams, reduce the system size and weight, and eliminate the alignment difficulties usually associated with conventional optical components. The most practical light source for commercial applications, that could include waveguide systems, is a laser diode (e.g., optical recording and printing). Since the standard input coupling grating would require the laser emission to remain within a range less than 1 nm, waveguide elements may not be considered in these types of applications. The operating range of readily available lasers can be as great as 20 nm because of the relatively broad emission spectrum and resulting wavelength fluctuations that occur from normal operating conditions (i.e. variations in the ambient temperature, selection of the emitted power, laser aging, etc.). An even greater range of input coupling (40 nm to 80 nm) is of interest for chemical and biochemical spectroscopic characterization. A major issue for including integrated optics in these types of applications is the sensitivity of the coupling process to a shift in wavelength and the ease of implementing a solution.
The range of input coupling has been successfully extended by adding refracting and diffracting surfaces before the waveguide grating or simply increasing the angular content of the incident beam. Spaulding and Morris in U.S. Pat. No. 5,010,458 added a prism to the surface of the waveguide grating, with a second grating on the upper surface of the prism, and demonstrated a full-width half-maximum (FWHM) input coupling efficiency of 12 nm. More recently, D. L. Hetherington, R. K. Kostuk, and M. C. Gupta, "Dispersion Compensation for an Integrated Optic Grating Coupler Utilizing a Transmission Volume Hologram", App. Opt., 32, 303-308 (1993) introduced the light through a tilted plate with a volume hologram to achieve a 5 nm FWHM input coupling efficiency. M.C. Gupta and L. Li, "Achromatic Compensation for Integrated Optic Grating Couplers with Focused Beams", Appl. Opt., 30, 1461-1463 (1991) recognized the angular sensitivity of the coupling process to the wavelength of light and extended the input coupling range to about 6 nm but lost input coupling efficiency.
An example of the utility of these concepts for use in an optical recording pick up device is presented in the disclosure by Jannson, Wang, Sun, and Strzelecki, "Broad-band Single-mode Coupler", Proc. OSA Annual Meeting, San Jose (1991). A second example, again pertaining to optical recording was disclosed by Kando et al, in "An Integrated Optical Pickup with Small Wavelength Aberrations", International Symposium on Optical Memory, IC-5, Sapporo (1991).